Titanium oxide (titania) has better properties than alumina in respect to hydrogenation ability, corrosion resistance, photooxidation ability, and the like and, for this reason, it is attracting attention as a high-performance catalyst material not only in the areas of hydrorefining of petroleum fractions and denitrification of waste gas but also recently in the area of photocatalysts. However, titanium oxide obtained by the conventional synthetic methods has a relatively small specific surface area and it is difficult to provide such titanium oxide with a pore structure suitable for given reactants. In addition to these problems, titanium oxide is inferior to alumina in mechanical strength.
Now, the pore structure, specific surface area, and mechanical strength of porous titanium oxide are decided by an aggregate of primary and secondary particles of titanium oxide and an article molded from such an aggregate is normally used as a catalyst or a catalyst carrier. The pore structure suitable for the reactants can be controlled by performing the pH swing operation and the specific surface area can be increased by adding a particle growth inhibitor, although not quite to a level as high as that of alumina or silica (JP 2003-40,689 A).
However, the mechanical strength of titanium oxide which occurs as spherical particles is generally lower than that of alumina which occurs as needle-shaped and/or column-shaped particles. Hence, an attempt has been made to raise the mechanical strength of titanium oxide by partially incorporating minute particles in ordinary particles (JP 2003-201,120 A); however, it is difficult to obtain mechanical strength suitable for use in general commercial catalysts even by this technique and, besides, the specific surface area of titanium oxide obtained in this manner cannot be said to be satisfactory.
Another method proposed in an attempt to increase the specific surface area comprises depositing titanium oxide and the like on the surface of a molded article of porous alumina by the chemical vapor deposition (CVD) technique [JP 3-275,143 A, JP 6-106,061 A, and “PPM,” September, 1992, published by The Nikkan Kogyo Shinbun Ltd.]. However, this technique deposits titanium oxide inside the fixed alumina pores and causes the following problem: the presence of anatase is confirmed by X-ray diffraction when the amount of deposited titanium oxide exceeds 13% and titanium oxide is induced to agglomerate to broaden the pore diameter distribution and decrease the number of effective pores most suitable to the target reactants. Furthermore, deposited titanium oxide blocks the alumina pores in some cases and this makes it difficult to deposit the desired catalyst metal in the ensuing step. Still further, when production on a commercial scale is contemplated, hydrogen chloride (HCl) forms inevitably from the reaction of titanium tetrachloride (TiCl4) with water (H2O) and this makes it necessary to devise some measures to prevent environmental pollution by hydrogen chloride.
To solve the aforementioned problems relating to titanium oxide, the following methods have been disclosed: molding of titanium oxide by the use of alumina, silica, silica/alumina, and the like as a binder and the use of a composite compound formed by co-precipitating titanium and the foregoing compounds as a catalyst carrier (JP 5-96,161 A, JP 5-192,575 A, JP 2000-135,440 A, JP 2001-9,279 A, and JP 10-118,495 A); the use of a composite oxide obtained by mixing gels as a catalyst carrier (JP 3-131,340 A and JP 5-184,921 A); and a sol-gel method [Materials Letters, 43, pp. 286-290 (2000) and J. Mater. Chem., (1994) 4 (4), pp. 585-589].
However, these techniques incur the possibility of producing the following undesirable effects: titanium oxide deteriorates in purity and becomes a substance merely showing an intermediate performance of titanium oxide and other substances and a composite effect due to substances other than titanium oxide promotes side reactions thereby lowering the selectivity of the reactants and accelerating deterioration of the catalyst.
Furthermore, a method for using a carrier obtained by depositing an aqueous titania solution on an alumina hydrogel followed by calcining is disclosed [JP 54-19,491 A, JP 2002-85,975 A, Symposium on Better Ceramics Through Chemistry (6th) 1994, pp. 445-450]. However, this technique merely deposits an aqueous solution of a mineral acid salt of titanium on an alumina hydrogel and, as a result, deposited titanium oxide assumes the anatase structure when analyzed by X-ray diffraction and is not chemically and/or microscopically united to the alumina. An agglomerate or an aggregate of titanium oxide forms partially and it is difficult to deposit titanium oxide over the whole surface of the alumina hydrogel.
Under the circumstances, the inventors of this invention have devoted every effort to develop porous titanium oxide which has a regulated pore structure, a large specific surface area, and excellent mechanical strength, found that layered porous titanium oxide which has a regulated pore structure, a large specific surface area, and excellent mechanical strength can be obtained by depositing titanium oxide on the surface of an inorganic oxide having a regulated pore structure and serving as a core while unevenly distributing titanium oxide on the surface of the inorganic oxide in conformity to a prescribed titanium localization index and maintaining the pore structure of the inorganic oxide, and completed this invention.
Accordingly, this invention provides layered porous titanium oxide which has a regulated pore structure, a large specific surface area, and excellent mechanical strength and is useful as a catalyst by itself or as a catalyst carrier.
Moreover, this invention provides a process for producing layered porous titanium oxide which has a regulated pore structure, a large specific surface area, and excellent mechanical strength and is useful as a catalyst by itself or as a catalyst carrier.
Still further, this invention provides a catalyst comprising layered porous titanium oxide which has a regulated pore structure, a large specific surface area, and excellent mechanical strength and useful as a catalyst for hydrorefining, hydrogenation of CO, denitrification of waste gas, photocatalysis, and the like.